Ionophore‐Based Biphasic Chemical Sensing in Droplet Microfluidics

Colorimetric detection of electrolyte ions in blood based on biphasic microdroplet extraction

BACKGROUND

Timely diagnosis and prevention of diseases require rapid and sensitive detection of biomarkers from blood samples without external interference. Abnormal electrolyte ion levels in the blood are closely linked to various physiological disorders, including hypertension. Therefore, accurate, interference-free, and precise measurement of electrolyte ion concentrations in the blood is particularly important.

RESULTS

In this work, a colorimetric sensor based on a biphasic microdroplet extraction is proposed for the detection of electrolyte ions in the blood. This sensor employs mini-pillar arrays to facilitate contact between adjacent blood microdroplets and organic microdroplets serving as sensing phases, with any color changes being monitored through a smartphone's colorimetric software. The sensor is highly resistant to interference and does not require pre-treatment of the blood samples. Remarkably, the sensor exhibits exceptional reliability and stability, allowing for rapid enrichment and detection of K+, Na+, and Cl- in the blood within 10 s (Cl-), 15 s (K+) and 40 s (Na+) respectively.

SIGNIFICANCE

The colorimetric sensor based on biphasic microdroplet extraction offers portability due to its compact size and ease of operation without the need for large instruments. Additionally, it is location-independent, making it a promising tool for real-time biomarker detection in body fluids such as blood.

Ion-modulated interfacial fluorescence in droplet microfluidics using an ionophore-doped oil

Fluorescence at the oil-water interface is used for chemical sensing in droplet microfluidics. Potassium ions in aqueous droplets are extracted into oil segments doped with an ionophore, a cation exchanger, and a cationic dye to expel the dye. When a low concentration of dye with a balanced solubility is used, it actively accumulates at the thin interface between oil and water instead of getting dissolved in the aqueous phase. The interfacial fluorescence is monitored distinct from the fluorescence in the oil sensor and the aqueous sample, allowing for highly sensitive and selective turn-on fluorescence sensing of ions.

Ultrasensitive Ionophore-Based Liquid Sensors for Colorimetric Ion Measurements in Blood

The self-monitoring of electrolytes using a small volume of capillary blood is needed for the management of many chronic diseases. Herein, we report an ionophore-based colorimetric sensor for electrolyte measurements in a few microliters of blood. The sensor is a pipet microtip preloaded with a segment of oil (plasticizer) containing a pH-sensitive chromoionophore, a cation exchanger, and an ionophore. The analyte is extracted from the sample into the oil via a mixing protocol controlled by a stepper motor. The oil with an optimized ratio of sensing chemicals shows an unprecedentedly large color response for electrolytes in a very narrow concentration range that is clinically relevant. This ultrahigh sensitivity is based on an exhaustive response mode with a novel mechanism for defining the lower and higher limits of detection. Compared to previous optodes and molecular probes for ions, the proposed platform is especially suitable for at-home blood electrolyte measurements because (1) the oil sensor is interrogated independent of the sample and therefore works for whole blood without requiring plasma separation; (2) the sensor does not need individual calibration as the consistency between liquid sensors is high compared to solid sensors, such as ion-selective electrodes and optodes; and (3) the sensing system consisting of a disposable oil sensor, a programmed stepper motor, and a smartphone is portable, cost-effective, and user-friendly. The accuracy and precision of Ca2+ sensors are validated in 51 blood samples with varying concentrations of total plasma Ca2+. Oil sensors with an ultrasensitive response can also be obtained for other ions, such as K+.

Magnetic Non-Spherical Particles Inducing Vortices in Microchannel for Effective Mixing

Mixing in microfluidic channels is dominated by diffusion owing to the absence of chaotic flow. However, high-efficiency microscale mixing over short distances is desired for the development of lab-on-chip systems. Here, enhanced mixing in microchannels achieved using magnetic nonspherical particles (MNSPs), is reported. Benefiting from the nonspherical shape of the MNSPs, secondary vortices exhibiting cyclical characteristics appear in microchannels when the MNSPs rotate under an external magnetic field. Increasing the rotation rate enlarges the secondary vortices, expanding the mixing zone and enhancing the mixing, resulting in a mixing efficiency exceeding 0.9 at Re of 0.069-0.69. Complementary micro-particle image velocimetry (µPIV) for flow field analysis clarifies the mixing mechanism. In addition, a chaotic vortex area is generated in the presence of two MNSPs, which shortens the distance required for achieving an appropriate mixing efficiency. This study demonstrates the potential of employing MNSPs as efficient mixers in lab-on-chip devices.

Facile microfabrication of three dimensional-patterned micromixers using additive manufacturing technology

This study investigates the manufacturing method of oblique patterns in microchannels and the effect of these patterns on mixing performance in microchannels. To fabricate three-dimensional (3D) and oblique patterns in microchannels, 3D printing and replica methods were utilized to mold patterns and microchannels, respectively. The angle and size of the patterns were controlled by the printing angle and resolution, respectively. The mixing efficiency was experimentally characterized, and the mixing principle was analyzed using computational fluid dynamics simulation. The analysis showed that the mixing channel cast from the mold printed with a printing angle of 30° and resolution of 300 μm exhibited the best mixing efficiency with a segregation index of approximately 0.05 at a Reynolds number of 5.4. This was because, as the patterns inside the microchannel were more oblique, “split” and “recombine” behaviors between two fluids were enhanced owing to the geometrical effect. This study supports the use of the 3D printing method to create unique patterns inside microchannels and improve the mixing performance of two laminar flows for various applications such as point-of-care diagnostics, lab-on-a-chip, and chemical synthesis.

Ion-Induced Phase Transfer of Cationic Dyes for Fluorescence-Based Electrolyte Sensing in Droplet Microfluidics

Fluorescence-based sensing in droplet microfluidics requires small sample volumes, allows for high-throughput assays, and does not suffer from photobleaching as each flowing sensor is only scanned one time. In this paper, we report a selective and sensitive fluorescence-based ion-sensing methodology in droplet microfluidics using a T-junction PDMS chip. The oil stream is doped with sensor ingredients including an ionophore, a cation exchanger, and a permanently cationic fluorophore as the optical reporter. Electrolyte cations from the aqueous sample are extracted into oil segments and displace the cationic dyes into aqueous droplets. Laser-induced fluorescence of the two immiscible phases is collected alternately, which is in clear contrast to most other ion-selective optode configurations such as nanoparticle suspensions that rely on mixed optical signals of two phases. The cation exchanger, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, is found to dramatically enhance the dye emission in the nonpolar sensing oil by preventing ion-pairing interactions and aggregations of the dye molecules, providing new insights into the mechanism of cationic dye-based ion sensors. The high dye brightness allows us to use low concentrations of sensing chemicals (e.g., 10 μM) in the oil and attain high sensitivity for detection of ions in an equal volume of sample. Using valinomycin as the ionophore and methylene blue as the dye, K⁺ is detected with a response time of ∼11 s, a logarithmic linear range of 10^-5 to 10^-2 M, a 20-fold total fluorescence response, >1000-fold selectivity against other electrolyte cations, and negligible cross-sensitivity toward the sample pH. The K⁺ concentration in untreated and undiluted whole blood and sweat samples is successfully determined by this microfluidic sensing method without optical interference from the droplet sample to the sensing oil. Detection of other ionic analytes can be achieved using the corresponding ionophores.

Characterization of the impact of mixing and droplet volumes on the behavior of microfluidic ion-selective droptodes

Droplet microfluidic optodes, or "droptodes", have emerged as a powerful technology for rapid detection of small ions in complex matrices. While using segmented aqueous phases provides the benefits of sample isolation, the influence of the liquid nature of the oil carrier phase has not yet been explored. In this paper, we examine the influence of microfluidic parameters on droptode efficiency, using potassium-sensitive droptodes as a model system. We found that while changing flow rates on device does not change droptode performance, both channel geometry and droplet size significantly impact droptode efficiency. Specifically, enhanced mixing of the droplets leads to faster equilibration on device and lowers limits of detection by about one order of magnitude. We also found that increasing the size of the sample droplet, at the expense of the size of the oil carrier/sensing phase, leads to higher sensitivity in the linear region of the droptode. These easily manipulated properties will allow one device to potentially be adapted for several different applications, based upon the type and concentration range of measurement required.

Microfluidics for Drug Development: From Synthesis to Evaluation

Drug development is a long process whose main content includes drug synthesis, drug delivery, and drug evaluation. Compared with conventional drug development procedures, microfluidics has emerged as a revolutionary technology in that it offers a miniaturized and highly controllable environment for bio(chemical) reactions to take place. It is also compatible with analytical strategies to implement integrated and high-throughput screening and evaluations. In this review, we provide a comprehensive summary of the entire microfluidics-based drug development system, from drug synthesis to drug evaluation. The challenges in the current status and the prospects for future development are also discussed. We believe that this review will promote communications throughout diversified scientific and engineering communities that will continue contributing to this burgeoning field.

Potentiometric detection of glucose based on oligomerization with a diboronic acid using polycation as an indicator

A novel potentiometric sensor for d-glucose (Glu) using 4,4'-biphenyldiboronic acid as a receptor and polyion (poly-N-(3-aminopropyl)methacrylamide, PAPMA) as an indicator is described. The diboronic acid condenses with Glu via its two cis-diol units to form cyclic or linear oligomeric polyanions which can interact electrostatically with PAPMA, thus efficiently decreasing its potentiometric response on a polycation-sensitive membrane electrode. Although d-fructose (Fru), d-galactose (Gal) and d-mannose (Man) show even higher binding affinities to the diboronic acid as compared to Glu, these monosaccharides with only one cis-diol unit cannot oligomerize with the receptor, which efficiently excludes the interferences from the Glu's stereoisomers. The results obtained from blood sample analysis indicate that the proposed sensor is promising for detection of Glu in real-world applications.

High-throughput screening by droplet microfluidics: perspective into key challenges and future prospects

In two decades of development, impressive strides have been made for automating basic laboratory operations in droplet-based microfluidics, allowing the emergence of a new form of high-throughput screening and experimentation in nanoliter to femtoliter volumes. Despite advancements in droplet storage, manipulation, and analysis, the field has not yet been widely adapted for many high-throughput screening (HTS) applications. Broad adoption and commercial development of these techniques require robust implementation of strategies for the stable storage, chemical containment, generation of libraries, sample tracking, and chemical analysis of these small samples. We discuss these challenges for implementing droplet HTS and highlight key strategies that have begun to address these concerns. Recent advances in the field leave us optimistic about the future prospects of this rapidly developing technology.

Quantifying plasticizer leakage from ion-selective membranes – a nanosponge approach

The spontaneous process of release of plasticizers from membranes typically used in ion-selective sensors is an effect which limits the lifetime of sensors and comes with a risk of safety hazards.